It has been the practice in the past in certain instances to use environmental air ducts and plenums to provide a passageway for electrical cables. The flame retardant characteristics of electrical cables conveyed through such ducts and plenums is necessarily extremely critical particularly with respect to flame proprogation and to the amount of smoke generated by the cable materials when exposed to a flame.
One test for evaluating flame propagation and smoke-generating characteristic of electrical cables for use in ducts and plenums (commonly called "plenum cable") is Underwriters Laboratories Inc., Tunnel Flame Test designated UL 910. The UL 910 Test is used by Underwriter Laboratories as a criterion for determining the suitability of particular cables for use in duct and plenums. The Test involves laying 24 foot lengths of the cable to be tested on supporting racks within the Test chamber and, after following a prescribed calibration procedure, exposing the samples to a 300,000 BTU methane flame for a prescribed period of time while measuring flame propagation and smoke generation in terms of specific optical density in accordance with the test procedures. The Underwriters Laboratories has established that, under the Tunnel Flame Test UL 910, plenum cables must exhibit a smoke characteristic in terms of a peak optional density of 0.5 or less and a maximum flame spread of 5 feet or less.
It is also desirable that the materials from which electrical plenum cable is made contain no halogens such as fluorine or chlorine which are known to generate a variety of toxic gases when heated above certain threshold temperature limits that may well be exceeded in a burning environment.
Up until the time of the present invention, plenum cable has almost exclusively utilized fluoroplastic material such as fluorinated ethylene propylene as a means of providing cable jacketing having low smoke generating characteristics in conjunction with flame retardancy characteristics. Such fluroplastic materials, however, are expensive and contain fluorine which in a burning environment are apt to release a complex series of potentially toxic or corrosive gases that could be conveyed through the ducts and plenums containing the cables.
In view of the above, a need exists for an inexpensive composition having attractive physical and electrical insulating characteristics that is adapted to char when exposed to a flame and is particularly suited for use in plenum cable by generating low smoke while otherwise meeting or exceeding the requirements of Underwriters Laboratories Tunnel Flame Test UL 910 whether or not any halogens are present in the composition.